1. Technical Field
The present invention relates to apparatus and method for automatically recognizing signals, particularly audio and video signals that may be transmitted via broadcast, computer networks, or satellite transmission. This has particular application in the detection of the transmission of copyright-protected material for royalty payment justification, and in the verification of transmission of scheduled programming and advertising.
2. Related Art
The need for automatic recognition of broadcast material has been established, as evidenced by the development and deployment of a number of automatic recognition systems. The recognized information is useful for a variety of purposes. Musical recordings that are broadcast can be identified to determine their popularity, thus supporting promotional efforts, sales, and distribution of media. The automatic detection of advertising is needed as an audit method to verify that advertisements were, in fact, transmitted at the times and for the duration that the advertiser and broadcaster agreed upon. Identification of copyright-protected works is also needed to assure that proper royalty payments are made. With new distribution methods, such as the Internet and direct satellite transmission, the scope and scale of signal recognition applications has increased.
Automatic program identification techniques fall into the two general categories of active and passive. The active technologies involve the insertion of coded identification signals into the program material or other modification of the audio or video. Active techniques are faced with two difficult problems. The inserted codes must not cause noticeable distortion or be perceivable to listeners and viewers. Simultaneously, the identification codes must be sufficiently robust to survive transmission system signal processing. Active systems that have been developed to date have experienced difficulty in one or both of these areas. An additional problem is that almost all existing program material has not yet been coded. The identification of these works is therefore not possible. For this reason we will dismiss the active technologies as inappropriate for many important applications.
Passive signal recognition systems identify program material by recognizing specific characteristics or features of the signal. Usually, each of the works to be identified is subjected to a registration process where the system “learns” the characteristics of the audio or video signal. The system then uses pattern-matching techniques to detect the occurrence of these features during signal transmission. One of the earliest examples of this approach is presented by Moon et al. in U.S. Pat. No. 3,919,479 (incorporated herein by reference). Moon extracts a time segment from an audio waveform, digitizes it and saves the digitized waveform as a reference pattern for later correlation with an unknown audio signal. Moon also presents a variant of this technique where low bandwidth amplitude envelopes of the audio are used instead of the audio itself. However, both of Moon's approaches suffer from loss of correlation in the presence of speed differences between the reference pattern and the transmitted signal. The speed error issue was addressed by Kenyon et al. in U.S. Pat. No. 4,450,531 (incorporated herein by reference) by using multiple segment correlation functions. In this approach, the individual segments have a relatively low time-bandwidth product and are affected little by speed variations. Pattern discrimination performance is obtained by requiring a plurality of sequential patterns to be detected with approximately the correct time delay. This method is accurate but somewhat limited in capacity due to computational complexity.
A video program identification system is described by Kiewit et al. in U.S. Pat. No. 4,697,209 (incorporated herein by reference). This system detects events such as scene changes to identify program changes. When a change is detected, a signature is extracted from the video signal and stored along with the time of occurrence. A similar process is performed at a central location for each available program source. Periodically the central site interrogates the stored data at the viewer location to obtain the signatures. These are compared to identify the changed program selection. This method has the advantage of only needing to select among a limited set of possibilities, but has the disadvantage that the queuing events that trigger signature extraction are not particularly reliable. Another video recognition system is described by Thomas et al. in U.S. Pat. No. 4,739,398 (incorporated herein by reference). The method discussed by Thomas identifies video programs by matching video features selected from a number of randomly selected locations in the frame sequence. The intensity, etc. of each location is quantized to one bit of resolution, and these bits are stored in a single word. A sequence of frame signatures is acquired from a program interval with the spacing of frame signatures selected according to a set of rules. Noisy or error prone bits within the signature words are masked. In the preferred embodiment there are eight frame signatures per interval each containing sixteen binary values. A key word is chosen from the frame signature set and is used to stage the pattern recognition process. When the key word is detected by bit comparison, a table of candidate patterns is accessed to locate a subset of patterns to be evaluated. These templates are then compared with the current video signature. Audio recognition is mentioned but no method is presented. Thomas also describes methods for compressing audio and video signals for transmission to a central location for manual identification. Corresponding video signatures are also transmitted. This allows the acquisition of unknown program material so that the new material can be added to a central library for later identification. The unknown signatures transmitted from the remote sites can be identified from templates stored in the central library or by manual viewing and listening to the corresponding compressed video and audio.
An audio signal recognition system is described by Kenyon et al. in U.S. Pat. No. 4,843,562 (incorporated herein by reference) that specifically addresses speed errors in the transmitted signal by re-sampling the input signal to create several time-distorted versions of the signal segments. This allows a high-resolution fast correlation function to be applied to each of the time warped signal segments without degrading the correlation values. A low-resolution spectrogram matching process is also used as a queuing mechanism to select candidate reference patterns for high-resolution pattern recognition. This method achieves high accuracy with a large number of candidate patterns.
In U.S. Pat. No. 5,019,899 Boles et al. (incorporated herein by reference) describe a video signal recognition system that appears to be a refinement of the Thomas patent. However, the method of feature extraction from the video signal is different. After digitizing a frame (or field) of video, the pixels in each of 64 regions is integrated to form super-pixels representing the average of 16×16 pixel arrays. Thirty-two pairs of super-pixels are then differenced according to a predefined pattern, and the results are quantized to one bit of resolution. As in the Thomas patent, a program interval is represented by eight frame signatures that are selected according to a set of rules. The pattern matching procedure involves counting the number of bits that correctly match the input feature values with a particular template. Boles also presents an efficient procedure for comparing the unknown input with many stored templates in real-time. For purposes of this invention, real-time operation requires all patterns to be evaluated in a thirtieth of a second.
Lamb et al. describe an audio signal recognition system in U.S. Pat. No. 5,437,050 (incorporated herein by reference). Audio spectra are computed at a 50 Hz rate and are quantized to one bit of resolution by comparing each frequency to a threshold derived from the corresponding spectrum. Forty-eight spectral components are retained representing semitones of four octaves of the musical scale. The semitones are determined to be active or inactive according to their previous activity status and comparison with two thresholds. The first threshold is used to determine if an inactive semitone should be set to an active state. The second threshold is set to a lower value and is used to select active semitones that should be set to an inactive state. The purpose of this hysteresis is to prevent newly occurring semitones from dominating the power spectrum and forcing other tones to an inactive state. The set of 48 semitone states forms an activity vector for the current sample interval. Sequential vectors are grouped to form an activity matrix that represents the time-frequency structure of the audio. These activity matrices are compared with similarly constructed reference patterns using a procedure that sums bit matches over sub-intervals of the activity matrix. Sub-intervals are evaluated with a several different time alignments to compensate for speed errors that may be introduced by broadcasters. To narrow the search space in comparing the input with many templates, gross features of the input activity matrix are computed. The distances from the macro features of the input and each template are computed to determine a subset of patterns to be further evaluated.
In U.S. Pat. No. 5,436,653 Ellis et al. (incorporated herein by reference) discuss a technique that seems to be a derivative of the Thomas and Boles patents. While the super-pixel geometry is different from the other patents, the procedures are almost identical. As in the Boles patent, super-pixels (now in the shape of horizontal strips) in different regions of a frame are differenced and then quantized to one bit of resolution. However, sixteen values are packed into a sixteen-bit word as in the Thomas patent, representing a frame signature. Potentially noisy bits in the frame signature may be excluded from the comparison process by use of a mask word. Frames within a program interval are selected according to a set of rules. Eight frame signatures of sixteen bits each are used to represent a program interval. As in the Thomas patent, one of the frame signatures is designated as a “key signature”. Key signature matching is used as a queuing mechanism to reduce the number of pattern matching operations that must be performed in the recognition process. Ellis addresses clumping of patterns having the same key signature as well as video jitter that can cause misalignment of superpixels. In addition, Ellis describes a method of using multiple segments or subintervals similar to the method described in the Kenyon et al. U.S. Pat. No. 4,450,531 patent. Unlike the Thomas and Boles patents, Ellis offers an audio pattern recognition system based on spectrogram matching. Differential audio spectra are computed and quantized to form sixteen one-bit components. Groups of these spectral signatures are selected from a signal interval. Ellis has updated this method as described in U.S. Pat. No. 5,621,454 (incorporated herein by reference).
Forbes et al. describe in U.S. Pat. No. 5,708,477 (incorporated herein by reference) a system that is used to automatically edit advertisements from a television signal by muting the television audio and pausing any VCR recording in progress. This is done by first detecting changes in the overall brightness of a frame or portion of a frame indicating a scene change. When a scene change is detected, a lowpass filtered version of the frame is compared with a similar set of frames that have been previously designated by the viewer to indicate the presence of an advertisement. When a match is detected, the audio/video is interrupted for an amount of time specified by the viewer when the segment was designated by the viewer as an advertisement. The detection decision is based on a distance metric that is the sum of the absolute values of corresponding input and template region differences. The intensity of various regions appears to be computed by averaging video scan lines. Forbes does not use any audio information or time series properties of the video.
While the inventions cited above in the prior art indicate progress in the technical field of automatic signal identification, there are a number of shortcomings in these technologies. To be accepted in the marketplace a system must have sufficient processing capacity to search simultaneously for a very large number of potential patterns from many different sources. The technologies of the prior art underestimate the magnitude of this capacity requirement. Further, if the capacity of the prior art systems is increased in a linear fashion through the use of faster processors, recognition accuracy problems become evident. These problems are in part due to the underlying statistical properties of the various methods, but are also caused by intolerance of these methods to signal distortion that is typical in the various media distribution and broadcast chains. Most of the cited inventions are designed to handle either audio or video but not both. None of the inventions in the prior art are capable of blending audio and video recognition in a simple uniform manner. While the duration of samples required for recognition varies among the different techniques, none of them is capable of recognizing a short segment from any part of a work and then moving to a different channel.
Thus, what is needed is a signal recognition system that can passively recognize audio and/or video data streams in as little as six seconds with great accuracy. Preferably, the system can recognize any portion of the input data stream, thus allowing channel-hopping as the system quickly recognizes one broadcast work and moves on to another.